Ranging is a process or method to determine distance from one device to another device. In the context of the present invention, ‘ranging’ refers to processes for directly measuring distance between a first transceiver and a second transceiver and for providing the measured distance information to a user of a transceiver device. Conventional radio ranging devices include Radio Distance and Ranging (RADAR) and Distance Measuring Equipment (DME). For purposes of this specification the term “ranging” is distinguished from the terms “positioning” and “locating” in that the latter terms refer to processes that determine location, in space, of a device. Alternatively, ‘positioning’ and ‘locating’ generally refer to determining a position of a first device relative to a position of at least one other device having a defined or known position.
Conventional ranging systems, for example DME systems, typically comprise a UHF transmitter/receiver (interrogator) in an aircraft and a UHF receiver/transmitter (transponder) on the ground. The aircraft interrogates the ground transponder with a series of pulse-pairs (interrogations). The ground station replies with an identical sequence of reply pulse-pairs characterized by a predetermined time delay, for example 50 microseconds. The DME receiver in the aircraft searches for pulse-pairs that match the individual aircraft's unique interrogation pattern.
When the aircraft interrogator matches a received pulse sequence with its unique interrogation sequence the interrogator locks onto the received signal. A radio pulse takes around 12.36 microseconds to travel one nautical mile (1.9 km) to and from, this is also referred to as a radar-mile. The time difference between interrogation and reply minus the 50 microsecond ground transponder delay is measured by the interrogator's timing circuitry and translated into a distance measurement in nautical miles which is then displayed in the cockpit.
Global positioning system (GPS) receivers determine location by measuring the propagation delay of specially constructed signals transmitted by GPS satellites. GPS signals comprise a relatively long Pseudo Random Noise (PRN) Coarse/Acquisition (C/A) code, a timing reference and sufficient data to support generation of a position solution by a GPS signal receiver. Propagation delay between GPS satellite and the GPS receiver is determined by applying code correlation techniques to the received GPS signal in the receiver to determine time of arrival (TOA) of the signal. The GPS receiver compares the TOA of the GPS signal to the transmitted timing reference portion of the signal. The time difference is used to calculate range from the receiver to the transmitting GPS satellite. The GPS receiver's position is determined by the geometric intersection of a plurality of simultaneously observed ranges from satellites with known coordinates in space.
GPS receivers work well for determining a receiver's location, i.e., position in space. However, a GPS range determination made by such receivers is carried out only as an intermediate calculation in a GPS location solution. A direct measurement of range between a user's GPS receiver and a transmitting satellite is not of interest to a typical user of a GPS receiver device. In other words a distance between a receiver and a GPS satellite with known coordinates is calculated only as part of an algorithm that relies on a plurality of such calculated distances to determine the receiver's position. In that sense, GPS systems are not ‘ranging’ systems, as the term ranging is defined in this specification, because GPS receivers cannot directly determine the distance between them.
Direct ranging from one device to another would be useful in many applications where distance from a reference device to a target device is of interest. For example, the ability to directly, rapidly and accurately determine distance from one fire-fighter to another, from a lost person to a rescuer, from a police officer to stolen property, etc. would provide a significant advantage in law enforcement, fire-fighting, and search and rescue operations. Therefore, portable ranging devices that can be carried on a person or affixed to an object would be desirable.
Conventional ranging systems like DME and RADAR are capable of directly determining range from one transceiver to another. However, they operate using specialized ranging circuits, ranging signals and ranging techniques. These circuits, signals and techniques are incompatible with the components and circuits used to carry out communication of information other than position information. For example, to provide DME-like circuits in a cellular telephone, a pulse pair generator and dedicated timing circuits would be required, in addition to providing typical communication circuits for functions like voice and text messaging. Such adaptation of existing communication devices to include conventional ranging circuits would impose significant burdens on, among other things, the device's operational efficiency, as well as device size, weight and power consumption.
It would be desirable to provide multi-purpose wireless communication devices capable of ‘ranging’, that is, of directly measuring distance between them. Further, it would be desirable to equip wireless communication devices used for video, text and audio communication with ranging capability, without the need for dedicated range timing circuits in these devices. It would further be desirable to provide ranging features in wireless communication devices while meeting bandwidth, power and other constraints imposed on wireless communications systems by international standards such as, for example, IEEE 802.15.4 and IEEE 802.11.
Ranging devices can be divided into two general categories, depending on how a device uses a received ranging signal in making a distance determination. A first category of devices relies on measurable properties of a received itself as indicators of distance of the signal transmitter from the signal receiver. For example, some ranging devices rely on received signal strength (RSS) as an indicator of relative range of a signal transmitting device with respect to the device receiving the transmitted signal. Another device in the first category measures bit error rate of information recovered from a received signal. Other devices in the first category include those that measure at least one of: phase, amplitude, phase and amplitude (I and Q), frequency and channel response of a received signal.
A second category of ranging devices determines distance based, at least in part, on determining a time of arrival (TOA) of a ranging signal at a distance measuring receiver. DME, as discussed above, is one example of devices falling into this second category. Impulse radio devices adapted for ranging, for example, Ultra Wideband (UWB) devices typically rely on TOA estimations of high-bandwidth ranging pulses to make distance determinations. For this second category of device, the accuracy of a device's distance measurement directly depends on the accuracy of at least one associated TOA measurement.
Therefore accuracy of a Time of arrival (TOA) estimate is an important consideration in implementing ranging functions. It would be desirable to have a communications transceiver capable of radio ranging and also capable of determining TOA with the highest possible accuracy. TOA estimation using ultra-wideband (UWB) signals provides a suitable ranging technique for indoor positioning applications which call for high levels of position accuracy. However, UWB systems have drawbacks including distance limitations, complexity and power consumption considerations associated with broad bandwidth signal processing.
Therefore, it would be desirable to provide ranging features in a narrowband wireless communication device. Further it would be desirable to achieve high accuracy in TOA measurements while meeting bandwidth, power and other constraints imposed on wireless communications systems by international standards such as, for example, IEEE 802.15.4 and IEEE 802.11.